The present invention relates to a delivery device in a sheet-fed offset rotary printing press, which is arranged on the upstream sheet convey direction side of a pile board and comprises a suction unit for decreasing a sheet convey speed.
In a sheet-fed offset rotary printing press of this type, a sheet printed by a printing unit is conveyed as it is gripping-changed from the grippers of an impression cylinder to the grippers of delivery chains. After that, the sheet is released from the grippers at the convey terminal end and drops onto a pile board to be stacked there. Since the sheet conveyed by the delivery chains is gripped by the grippers only at its leading edge, the trailing edge of the sheet may flutter. Also, when the sheet is released to drop, an inertia occurs as the sheet travels, and the edge of the sheet may not be aligned when stacked.
In order to prevent this, a plurality of suction wheels line up below the sheet under conveyance on the upstream sheet convey direction side of the pile board in the widthwise direction of the sheet. The suction wheels have suction surfaces which draw the sheet by suction in slidable contact with it and rotate at a peripheral speed lower than the sheet convey speed. Thus, the traveling speed of the released sheet that has been gripped by the grippers is decreased. In double-sided printing, if the suction wheels described above are located within a pattern printed on the reverse surface of the sheet, the suction surfaces of the suction wheels damage the image portions printed on the sheet to degrade the printing quality. Hence, the suction wheels must be located in non-image portions which are not printed.
If non-image portions do not exist other than the two ends of the sheet in the widthwise direction or the number of non-image portions is small, the number of suction wheels is limited, and the center of the sheet becomes slack between the suction wheels, that is, so-called middle slack occurs. When such middle slack occurs, the two ends of the sheet may be disengaged from the suction wheels and are not drawn by them by suction, so the sheet convey speed cannot be sufficiently decreased. As a result, the sheet flutters. When the sheet is stacked, the edge of the sheet is not aligned well, and comes into contact with the brackets of the suction wheels to damage the printing surface.
In order to solve this, an apparatus is proposed as shown in Japanese Patent Laid-Open No. 2000-95409, which comprises a plurality of suction wheels which are arranged in the widthwise direction of a sheet to be conveyed, and at least a pair of nozzles which are arranged below the sheet on the two sides of the sheet to sandwich the center of the sheet in the widthwise direction. The pair of nozzles discharge air to blow upward the sheet under conveyance. In this apparatus, the air discharge directions from the nozzles are directed outwardly in the widthwise direction of the sheet to correct the middle slack, in which the sheet becomes slack downward, by an air layer formed by air from the nozzles. Thus, the two ends of the sheet are not disengaged from the suction wheels.
In the suction device of the conventional sheet-fed offset rotary printing press, air is blown to the sheet to pull the two ends of the sheet outwardly in the widthwise direction, thus stretching the sheet tightly. To lift the sheet not partially but entirely uniformly, the air blowing amount must be adjusted. It is, however, difficult to adjust the air blowing amount, and this adjustment takes time. In order to prevent middle slack of the sheet, nozzles must be provided in addition to the suction wheels. Accordingly, a hose which supplies discharge air to the nozzles is necessary in addition to a hose that supplies suction air to the suction wheels. This leads to a complicated structure and increases the manufacturing cost.